The casting of a two-component, clear polyurethane resin upon a substrate to produce a decorative emblem is well known in the art. Cast polyurethane, when cured, gives a lens effect to the surface applied as described in U.S. Pat. Nos. 4,100,010 and RE 33,175. These cast polyurethane resins are commonly referred to as “doming” or “lensing” resins. Doming or lensing resins are typically clear, colorless, high gloss, room temperature or elevated temperature curing, thermosetting systems developed to provide aesthetic enhancement and environmental protection to objects such as, but not limited to, paper, plastic, wood, metal, labels, decals, plaques, badges, name plates, lapel pins, automotive ornamentation, and automotive dashboards to form a durable three-dimensional coating or dome.
In order for a liquid doming resin to achieve the required appearance on an object once it is cured, it preferably has a number of characteristics intrinsic to both the liquid components and the cured resin. First, the formulation should be a clear, colorless, low viscosity liquid. It should flow sufficiently to cover the entire surface to which it is applied. It should produce a coating from approximately 20 mils (0.5 mm) to 100 mils (2.5 mm) high. It should fully cure within forty-eight hours at 25° C. & 50% R.H. The curing of the coating resin formulation should not cause shrinkage, wrinkles, surface defects, curling, or other deviations from a clear, transparent, smooth, high gloss surface. It should not contain volatile solvents (less than 1%). Once cured, the coating should maintain its initial hardness and flexibility after heat and environmental aging.
Doming resins typically have an application viscosity of about 100 to 5000 cps, are generally clear and colorless, and cure to a smooth, defect free, flexible or rigid coating. In addition to protecting the substrate surface from the environment, the resins are usually transparent providing an aesthetically appealing lensing effect to the pattern, design, or scripting on or near the substrate surface over which the resins are placed.
Currently, conventional doming resins are practically applied as two-component, 100% solids, polyurethane systems, which may be room temperature cured or cured with heat. Two-component epoxy systems are sometimes used in indoor applications. However, they cannot be used outdoors or in applications where they will be exposed to a high concentration of UV light, as they will yellow. Polyurethane systems, based primarily on aliphatic diisocyanates, are used for most indoor and outdoor applications.
There are major disadvantages with these two-component conventional systems. They need specialized two-component metering-mixing and dispensing systems to accurately dispense and mix the two reactive components. The curing of these resins must be made sufficiently slow so that the resin does not cure in the mixing unit. However, the resins then cure slowly once dispensed onto the desired substrate. Smaller parts cure slower than larger parts. Tack-free times are usually greater than six hours at ambient temperatures with full cure taking up to five days. Vacuum tables are often used to keep the substrate flat until the resin thickens sufficiently so that it will not flow off of the part. This can take hours at ambient temperatures.
Special safety precautions are also necessary due to the inherent toxicity of isocyanate and epoxy resin components in these formulations. Additionally, in isocyanate-functional systems, outgassing can occur when the isocyanate component undesirably reacts with a source of water or carboxyl groups and not the hydroxyl groups present in the first component. This will cause carbon dioxide bubbles to generated & become entrapped in the cured product, essentially ruining the appearance and protective properties of the coating.
Compounds based on the heavy metals; especially mercury and lead, are commonly used as catalysts in these polyurethane based systems. These compounds present both safety and environmental concerns. Phenyl mercuric acetate is one example of such a catalyst.
Furthermore, polyurethane-based systems cannot be made softer than about 70 Shore A. Thus, they cannot be used on pressure sensitive adhesive labels for applications requiring a very soft, flexible dome, such as on highly curved surfaces, unless very strong adhesives are used.
These two-component polyurethane and epoxy systems are also used in molding applications to produce molded designs, nameplates, and lettering and in scripting and design applications. Here again these systems are limited by their long curing times and usually must be left to cure at least twenty-four hours before they can be packaged.
Heat curing systems to accelerate the curing process are available. However they are expensive and require at least twenty minutes at 60° C. for the epoxies and polyurethanes to reach a tack free state. After that, they again must sit for at least twenty-four hours before the molded parts and letters can be packaged.
One-component, thin coatings that cure by actinic radiation are extensively used for thin coatings. These coatings are usually applied at less than 5 mils thick (0.13 mm). High build (>10 mils) UV curable coatings are severely limited because of high shrinkage during the curing process. Because of this high shrinkage these coatings have poor flexibility and poor adhesion to many substrates. This high shrinkage also limits the use of these UV curing doming resins to very small parts, usually less than 3 cm diameter. If placed on larger parts, such as flexible labels or decals, the shrinking would cause the part to curl on curing. Furthermore, these systems yellow on exposure to UV light and cannot be used in outdoor applications. Furthermore, acrylic monomers are very expensive and these systems contain high acrylic monomer contents. There are also health concerns over the toxicity and sensitizing properties of the acrylate based functional monomers.
Co-pending application Ser. No. 11/124,077 describes one-component silylated, high build coatings that can also be used in doming resin applications. These silylated systems have many advantages over two-component systems. They do not require two-component, meter-mix-dispensing systems, they do not produce carbon dioxide bubbles on exposure to moisture and they do not contain heave metals. Although the application is useful for many applications, the tack-free times are in the area of thirty minutes to two hours.
Dual cure compositions have been used for structural adhesives such as described in EP 646632A1, WO 0105846 and WO 98/53008. A photocurable resin composition for use in coatings comprising a component with dual functionality, acrylate and silane, is described in EP 0549228B1.
There is a need for a curing system that can be used in doming, scripting, and molding applications that cures quickly or instantly, does not shrink, has good UV light and weathering resistance, and can be packaged as soon as it has been cured.